Method of detecting combustion misfires by evaluating RPM fluctuations

ABSTRACT

A method is introduced for detecting combustion misfires on the basis of the time-dependent course of the rotational movement of a transducer wheel, which is coupled to the crankshaft of an internal combustion engine. In this method, the segment times, in which predetermined segments of the transducer wheel pass a sensor, are detected and processed to a measure for the rough running of the internal combustion engine. A conclusion as to misfires is drawn from the behavior of the measure. The measure for the rough running is formed by digital filtering of segment times with predetermined filter coefficients.

IN THE DISCLOSURE

The invention relates to a method for detecting combustion misfires ininternal combustion engines.

BACKGROUND OF THE INVENTION

Combustion misfires lead to an increase of toxic substances emittedduring operation of the engine and can, in addition, lead to damage of acatalytic converter in the exhaust-gas system of the engine. A detectionof combustion misfires in the entire rpm and load ranges is necessary tosatisfy statutory requirements as to onboard monitoring of exhaust-gasrelevant functions. In this context, it is known that, during operationwith combustion misfires, characteristic changes occur in the rpm curveof the engine compared to normal operation without misfires. Normaloperation without misfires and operation with misfires can bedistinguished from a comparison of these rpm curves.

A method operating on this basis is already known from German patentpublication 4,138,765.

In this known method, a crankshaft angular region which is characterizedas a segment is assigned to each cylinder. The segments are realized,for example, by markings on a transducer wheel coupled to thecrankshaft. The segment time in which the crankshaft passes through thisangular region is dependent, inter alia, upon the energy converted inthe combustion stroke. Misfires lead to an increase of the segment timesdetected in synchronism with the ignition. According to the knownmethod, a criterion for the rough running of the engine is computed fromthe differences of the segment times. In addition, slow dynamicoperations such as the increase of the engine rpm for a vehicleacceleration are mathematically compensated. A rough-running value whichis computed in this way for each ignition, is likewise comparedignition-synchronously to a predetermined threshold value. Exceedingthis threshold value is evaluated as a misfire. The threshold value isdependent, as may be required, from operating parameters such as loadand engine speed (rpm).

The reliability with which misfires can be detected with this methoddrops naturally to a greater extent the less individual misfires operateon the rpm of the crankshaft. The reliability of the misfire detectiontherefore drops with increasing number of the cylinders of the engineand with increasing rpm as well as decreasing load.

SUMMARY OF THE INVENTION

In view of this background, the object of the invention is to provide amethod which further improves the reliability of the misfire detectionin internal combustion engines having a high number of cylinders even athigh rpm and low loads.

The invention is based upon the realization that the segment timeincrease as a consequence of a misfire at high rpm is no longerdistributed to one segment time but instead to several segment times.

An element of the invention comprises undertaking an evaluationincluding a greater crankshaft angle. This takes place, for example, byconsidering additional segment times before and after a possiblemisfire. Various computation rules have been shown to be advantageousfor the computation of a rough-running quantity while consideringfurther segment times. It is common for these rules that they aredefined by discrete convolution in the signal processing stage andfeature extraction stage by means of a digital filter and predeterminedfilter coefficients.

These various rules can be expressed without dynamic corrections withthe following formula: ##EQU1## wherein: b(n) are filter coefficients,Ma, Me are filter start and filter end, respectively, and M=Ma-Me+1 isthe filter length and * is the convolution operator.

Multiplication by the reciprocal value of ts(n)³ as a proportionalityfactor operates to eliminate the dependency upon rpm.

In lieu of the segment times, the speeds ω, which are allocated to thesegments, can be used: ##EQU2##

It is an essential feature of the invention that the methods fordetecting misfires can be supplemented by methods for dynamiccompensation. The object is to eliminate the influence of misfire-likerpm changes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawingswherein:

FIG. 1 shows the technical background of the invention;

FIG. 2 defines a computer suitable for carrying out the method of theinvention;

FIGS. 3a-3c explains the known principle for forming segment times as abasis of a measure for the rough-running on the basis of rpmmeasurements;

FIG. 4 shows a function block diagram of a digital filter having limitedpulse response as an embodiment of the invention; and,

FIGS. 5a-5c show the effect of the invention based on signal traces as afunction of time.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 shows an internal combustion engine 1 having an angle transducerwheel 2 having markings 3 as well as an angle sensor 4, a block 5, whichserves for signal processing and feature extraction, and a block 6.Block 6 functions to detect combustion misfires by evaluating thefeature signals on the basis of threshold value comparisons, neuronalnetworks, fuzzy classifiers or even other known means and/or methods.Operating parameters of the engine such as load and rpm are supplied, asrequired, to the blocks 5 and 6. If combustion misfires are determined,then, for example, a warning lamp 7 can be switched on.

The essence of the invention concerns block 5, that is, the signalprocessing and feature extraction and/or obtaining the signal q(n) inFIG. 1. The function blocks 5 and 6 are preferably realized byprogramming a computer 8 having a basic function which is shown in FIG.2. According to this basic function, a computer unit 2.1 arbitratesbetween an input block 2.2 and an output block 2.3 utilizing programsand data stored in a memory 2.4.

The angle transducer wheel is coupled to the crankshaft of the engine.The rotational movement of the angle transducer wheel is converted intoan electrical signal with the aid of the angle sensor 4 which isrealized as an inductive sensor. The periodicity of the electricalsignal defines an image of the periodic passing of the markings 3 at theangle sensor 4. The time duration between an increase and a drop of thesignal level therefore corresponds to the time in which the crankshafthas rotated further over an angular region corresponding to the extentof a marking.

These time durations are processed further in the control apparatus 8 toa measure q(n) for the rough running of the engine. The controlapparatus 8 is realized as a computer.

FIG. 3a shows a subdivision of the angle transducer wheel into foursegments wherein each segment has a predetermined number of markings.The marking OTk is assigned to that top dead center of the pistonmovement of the k-th cylinder of an internal combustion engine (in thisembodiment, an eight-cylinder engine), which lies in the combustionstroke of this cylinder. A rotational angular region Φ_(k) is definedabout this point and extends in this embodiment over one quarter of themarkings of the angle transducer wheel. In the same manner, angularregions Φ₁ to Φ₈ are assigned to the combustion strokes of the remainingcylinders with a four-stroke principle being assumed wherein thecrankshaft rotates twice for each complete work cycle. For this reason,the region Φ₁ of the first cylinder corresponds to the region Φ₅ of thefifth cylinder and so on. The angular regions, which correspond to onecrankshaft revolution, can be separated from each other, can follow eachother directly or can overlap each other. In the first case, markingsare provided which are not assigned to any angular region. In the secondcase, each marking is allocated precisely to one angular region and, inthe third case, the same markings can be assigned to different angularregions. Any desired lengths and positions of the angular regions aretherefore possible.

In FIG. 3b, the times ts are plotted in which the angular regions arepassed through with the rotational movement of the crankshaft. Here, amisfire in cylinder k is assumed. The absence of torque connected withthis misfire leads to an increase of the corresponding time span ts. Thetime spans ts then already define a criterion for the rough runningwhich is, in principle, suitable for detecting misfires. By a suitableprocessing of the time spans ts, the rough-running value receives thedimension of an acceleration and has an improved signal/noise ratio ashas been shown empirically. The suitable processing is especiallyperformed by forming the differences:

    ts(n+1)-ts(n)

of mutually adjacent time spans and normalizing these differences to thethird power of the time span ts(i) to an ignition stroke having index i.

FIG. 3c shows the influence of rpm changes on the detection of the timedurations ts. The case of a reduction in rpm is shown as it typicallyoccurs during overrun operation of a motor vehicle. This effect becomesmanifest in a relatively uniform extension of the detected times ts. Tocompensate for this effect, it is, for example, known to form anadditive corrective term D for dynamic compensation and, when therough-running value is computed, to so consider this term D that theextension effect is compensated.

All of the following examples are with respect to a 12-cylinder enginebut can be converted to combustion engines having other numbers ofcylinders. A known feature value for a 12-cylinder engine is computedwhile eliminating the dependency on rpm by dividing by ts(n)³ anddynamic compensation to form: ##EQU3##

An embodiment of the invention is realized by the digital filter shownin FIG. 4.

Twelve memory cells are shown in the upper row of FIG. 4 into whichtime-discrete sequential values of segment times ts are written. Foreach of the inputs of a segment time ts taking place from the left, thecontents of the right memory cells are written over by the content ofthe next adjacent memory cells on the left-hand side. The content ofeach memory cell 1 to 12 in the center row of FIG. 4 is logicallycoupled multiplicatively to a filter coefficient b1 to b12 after thetime discrete renewal. Each filter coefficient having index i isallocated to a memory cell having index i. The results of these logiccouplings are summed in the lower row to the rough-running feature valueq(n). Stated otherwise, each segment time is pushed through thearrangement from left to right and weighted in a time-discrete mannerwith various filter coefficients. The filtering shown corresponds to adigital filter having a limited pulse response.

The number 12 as a number of the memory cells assumes a 12-cylinderengine and filtering over one camshaft revolution. A filtering over arange of up to approximately two camshaft revolutions appears to bepurposeful.

The result of this digital filtering for the case of a correlation ofthe misfire-typical increase in the segment time signal having a sinefunction over a complete camshaft revolution is shown in FIG. 5.

The variable rough-running feature signal q results from: ##EQU4##wherein: filter coefficients b=(sin(2πm/12); Ma≦m≦Me; Ma=-5; and, Me=6.

FIG. 5a shows the sequence of the unfiltered segment times ts and FIG.5b defines the filter coefficients and FIG. 5c shows the time-dependenttrace of the rough-running feature variable qn as a result of thetime-discrete and value-discrete logic coupling of the segment timeswith the filter coefficients. The improved signal/noise ratio ofq(n)-values compared to the unfiltered segment times is obvious.

A further possibility is the correlation of the misfire-typical increasein the segment-time signal having a jump function: ##EQU5## wherein:b=(1 1 1 1 1 1 -1 -1 -1 -1 -1 -1); Ma=-5; and, Me=6.

A forming rule, which is denoted as segment-time curving, determines thedeviation of the segment times from a straight line which is drawnbetween two segment time points lying at a distance from each other:##EQU6## wherein: b=(-33/12 0 0 1 1 1 1 1 1 0 0 0 (33/12-6)); Ma=-7;and, Me=5.

A further embodiment arises by forming the difference between twodifferent filter outputs. An example is the difference between afiltering over 360° and 720° crankshaft angle: ##EQU7## wherein: b=1/12(-1 -1 -1 1 1 1 1 1 1 -1 -1 -1); Ma=-6; and, Me=5.

The slower filter effects the dynamic compensation for the differencebetween differently rapid filters. In addition to the form alreadyshown, the following form is also possible: ##EQU8## wherein: b=1/12 (11 1 1 1 1 -1 -1 -1 -1 -1 -1); Ma=0; and, Me=11.

As a further alternative, a complex exponential function can beutilized. The result is complex. The phase information, which is presentin addition to the magnitude, facilitates the cylinder identification.##EQU9## wherein: b=exp(j2π/12); Ma=-6; and, Me=5.

Additional embodiments evaluate the increase over previously averagedsegment times. The averaging can take place over one revolution of thecrankshaft. Then, q₂ (n) results. For averaging over one camshaftrevolution, one obtains: ##EQU10## wherein: b=(1 1 1 1 1 1 1 1 1 1 1 1-1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1);

Ma=-12; and, Me=11.

In addition, the statement of these features can be supplemented by theheight of the segment time drop. The increase and the decrease within acrankshaft revolution of average segment times can be considered:##EQU11## wherein: b=(-1, -1, -1, -1, -1, -1 2, 2, 2, 2, 2, 2

-1, -1, -1, -1, -1, -1);

Ma=-9; and, Me=8.

As an alternative, the increase and decrease within one camshaftrevolution can be considered: ##EQU12## wherein: b=(-1 -1 -1 -1 -1 -1 -1-1 -1 -1 -1 -1 2 2 2 2 2 2 2 2 2 2 2 2

-1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1);

Ma=-17; and, Me=18.

An evaluation encompassing a greater crankshaft angular range issuccessful in a further embodiment from: ##EQU13## wherein: b=(-5/3 0 01 1 1 1 0 -1 -1 -1 -1 5/3); Ma=-7; and, Me=5.

A further smoothing results from: ##EQU14##

The above-mentioned feature signals q₁, q₂, q₅, q₆, q₇ and q₈ can besupplemented in additional embodiments of the invention by measures fordynamic compensation. The object of these measures is to compensate theinfluence of rpm changes which, for example, are caused by a drop orincrease of the mean engine rpm during driving operation with vehicleacceleration (FIG. 3c).

The slow filter effects the dynamic compensation for the rough-runningfeature quantities q_(4a) and q_(4b). The segment-time curvature q₃already contains a dynamic compensation as do q₉ and q₁₀.

The methods for dynamic compensation are subject to a coarsesubdivision. A first method is applied to signals which arise because ofprevious difference formation in the segment-time signal or in thefiltered segment-time signal.

If one proceeds from linear increases of the segment times as aconsequence of acceleration, then with: ##EQU15## a signal for rpm trendelimination can be derived which is changed into q₉ (n).

A second method is applied directly to the segment-time signal or thefiltered segment-time signal. In principle, all signals are suitable fordynamic compensation which arise via lowpass formation from thesegment-time signal. One example comprises the mean-value formation over12 segment-time values: ##EQU16## Furthermore, the possibility exists touse a median filter on the segment-time signal for dynamic compensation:##EQU17## The computation of the signals for dynamic compensation cantake place via digital filters. For the known examples, the followingfilter coefficients result:

Trend elimination dk₁ (n):

b=1/12 (1 0 0 0 0 0 0 0 0 0 0 0 -1);

Ma=-7; and, Me=5.

Lowpass filtering dk₂ (n):

b=1/12 (1 1 1 1 1 1 1 1 1 1 1 1);

Ma=-6; and, Me=5.

The median filter requires a nonlinear algorithm.

The methods for misfire detection can be taken together with those ofdynamic compensation in a filter function.

We claim:
 1. A method for detecting combustion misfires on the basis ofthe time-dependent course of the rotational movement of a transducerwheel, which is coupled to the crankshaft of an internal combustionengine, the method comprising the steps of:detecting one of thefollowing:(a) the segment times (ts(n)) during which predeterminedsegments of the transducer wheel pass a sensor; and, (b) the mean enginespeeds (rpm) which are assigned to the segments of the transducer wheel;forming a measure for the rough running of the engine by digitallyfiltering the segment times with predetermined filter coefficients;drawing a conclusion as to misfire from the behavior of said measure;said filtering detecting an angular range up to four camshaftrevolutions and said filtering taking place via a filter having limitedpulse response; said filtering taking place in correspondence to thefollowing rule: ##EQU18## wherein: b(n) are filter coefficients; Ma, Meare filter begin and filter end; M=Ma-Me+1 is filter length; * isconvolution operator; n=numbers the ignition strokes in sequence;ts=segment times; q=rough-running feature signal; q(n)=rough-runningfeature signal for ignition stroke (n); ts(n)=segment times; and,b=filter coefficients with which the segment times ts(n) are weighted inthe digital filtering thereof.
 2. A method for detecting combustionmisfires on the basis of the time-dependent course of the rotationalmovement of a transducer wheel, which is coupled to the crankshaft of aninternal combustion engine, the method comprising the steps of:detectingone of the following:(a) the segment time (ts(n)) during whichpredetermined segments of the transducer wheel pass a sensor; and, (b)the mean engine speeds (rpm) which are assigned to the segments of thetransducer wheel; forming a measure for the rough running of the engineby digitally filtering the segment times with predetermined filtercoefficients; drawing a conclusion as to misfire from the behavior ofsaid measure; said filtering detecting an angular range up to fourcamshaft revolutions and said filtering taking place via a filter havinglimited pulse response; said filtering taking place in correspondence tothe following rule: ##EQU19## wherein: b(n) are filter coefficients; Ma,Me are filter begin and filter end; m=Ma-Me+1 is filter length; * isconvolution operator; ω(n)=angular speeds of crankshaft within specificsegment times ts(n); n=numbers the ignition strokes in sequence;ts=segment times; q=rough-running feature signal; q(n)=rough-runningfeature signal for ignition stroke (n); ts(n)=segment times; and,b=filter coefficients with which the segment times ts(n) are weighted inthe digital filtering thereof.
 3. The method of claim 1, whereinfunction values of a complex exponential sine function or real sinefunction are used as filter coefficients.
 4. The method of claim 1,wherein filter coefficients are used which are a measure for thedeviation of the segment times from a straight line, which is drawnthrough two segment-time points lying at a distance from each other. 5.The method of claim 1, wherein the function values of a step functionare used as filter coefficients.
 6. The method of claim 1, whereinfilter coefficients are used which define a difference in speeds withwhich the output quantity of the filter reacts to input changes at theinput end thereof.